remote_storage config: move handling of empty inline table {} to callers (#8193)

Before this PR, `RemoteStorageConfig::from_toml` would support
deserializing an
empty `{}` TOML inline table to a `None`, otherwise try `Some()`.

We can instead let
* in proxy: let clap derive handle the Option
* in PS & SK: assume that if the field is specified, it must be a valid
  RemtoeStorageConfig

(This PR started with a much simpler goal of factoring out the
`deserialize_item` function because I need that in another PR).

libs/utils/src/lib.rs

@@ -94,6 +94,8 @@ pub mod env;
 
 pub mod poison;
 
+pub mod toml_edit_ext;
+
 /// This is a shortcut to embed git sha into binaries and avoid copying the same build script to all packages
 ///
 /// we have several cases:

libs/utils/src/toml_edit_ext.rs

@@ -0,0 +1,22 @@
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("item is not a document")]
+    ItemIsNotADocument,
+    #[error(transparent)]
+    Serde(toml_edit::de::Error),
+}
+
+pub fn deserialize_item<T>(item: &toml_edit::Item) -> Result<T, Error>
+where
+    T: serde::de::DeserializeOwned,
+{
+    let document: toml_edit::Document = match item {
+        toml_edit::Item::Table(toml) => toml.clone().into(),
+        toml_edit::Item::Value(toml_edit::Value::InlineTable(toml)) => {
+            toml.clone().into_table().into()
+        }
+        _ => return Err(Error::ItemIsNotADocument),
+    };
+
+    toml_edit::de::from_document(document).map_err(Error::Serde)
+}